The phrase "passive preamp" comes close to being an oxymoron. The idea seems attractive at first, but in the real world it bogs down in difficulties. Is it impossible? Of course not. Lots of people use them.
The problem is that you either go with a high resistance in order not to load down the output of your source device--in which case you have a high output resistance from your control which can cause all sorts of problems
--or--
you go with a low resistance and find that the output of your source can't deliver.
Either way, the sound suffers.
It's a pain in the rump, but a better solution is to hack into the source and put a volume control upstream of the output so that you can have a low Zout without straining the output stage.
The best solution is just to go ahead and use an active preamp. For all their faults, they're still the best approach. If you want a truly minimalist preamp, just use a follower. You'll have no gain, but you'll have a high Zin, low Zout, and plenty of current to deal with cable capacitance etc.
Grey
The problem is that you either go with a high resistance in order not to load down the output of your source device--in which case you have a high output resistance from your control which can cause all sorts of problems
--or--
you go with a low resistance and find that the output of your source can't deliver.
Either way, the sound suffers.
It's a pain in the rump, but a better solution is to hack into the source and put a volume control upstream of the output so that you can have a low Zout without straining the output stage.
The best solution is just to go ahead and use an active preamp. For all their faults, they're still the best approach. If you want a truly minimalist preamp, just use a follower. You'll have no gain, but you'll have a high Zin, low Zout, and plenty of current to deal with cable capacitance etc.
Grey